Endothermic chemical reactions require energy. Energy can be supplied in numerous ways. Frequently, photons of various wavelengths are employed.
Since these reactions occur at relatively low temperatures, catalysts are added to accelerate the reaction.
In the following, this combination of inducing chemical reactions by photons and increasing the speed of reactions by catalysts will be referred to as photocatalysis.
Photocatalyzed chemical reactions have been the subject of increasing attention for some time. Applications have been sought for these reactions in the area of environmental protection. In Environmental Sci. Technol. Vol. 17, No. 10 (1983) 828-31, for example, a reaction is described in which chloroform is degraded in an aqueous suspension of titanium dioxide by photoassisted, heterogeneous catalysis.
Chloroform is a main by-product occurring in the disinfecting chlorine treatment of drinking water and also, for example, water in swimming pools.
Chloroform is ascribed carcinogenic characteristics. This method is applicable to aqueous suspensions with high particle content, known as slurries. A disadvantage of this method is the production of hydrochloric acid, which must be removed from the system. The method works with slurries if chlorine containing mineralization products are formed. The inventive process, however, overcomes all these difficulties with the involved washing process as described below.
European Patent Application 89100265.1 discloses a method and apparatus for the removal of hydrogen sulfide from exhaust gases using a titanium dioxide catalyst applied on the surface of a honeycomb support of activated charcoal, and ultraviolet radiation.
The activated charcoal adsorbs odorous compounds, such as H.sub.2 S and scatoles, which are degraded by ultraviolet light. The method is a selective method and has the disadvantage that only part of the total surface can be utilized for the reaction.
In DOS 40 23 995.0, a method is disclosed in which pollutants are photocatalytically mineralized by a fixed-bed catalyst which is moved relative to the photon source.
Compared to other methods for the destruction of pollutants, such as incineration, this method has the advantage that it does not create additional environmental pollutants. These occur in incineration when auxiliary fuel is required, which usually results in the production of carbon dioxide.
The method described in the cited DOS patent does not require auxiliary fuel, even when the pollutant occurs in very low concentrations, and is carried out at relatively low temperatures. Liquid or solid mineralization products such as phosphoric and arsenic acid, however, can accumulate on the photocatalyst if pollutants are continually employed which, in addition to carbon, hydrogen, oxygen, and nitrogen, contain, e.g., halogens, sulfur, phosphorus, or arsenic, as well as pollutants and other wastes consisting solely of these latter elements. The low reaction temperature, generally below 50.degree. C., does not allow these substances to evaporate. As a result, the photocatalytic function is increasingly inhibited, and the reaction finally stops completely.
There are several known methods for removing pollutants and contaminants from exhaust gases and other media in which they are undesirable.
These include purely physical methods such as adsorption or absorption, or purely chemical methods such as chemisorption or incineration.
These methods, however, are frequently unsuitable for mineralizing or removing complicated chemical compounds, such as sulfur hexafluoride, by economically or ecologically acceptable means. These methods have other disadvantages as well. Physical sorption methods are limited by sorbent capacity. The loaded sorbent must either be disposed of, frequently as hazardous waste, or be desorbed, a process requiring large amounts of energy. The alternative incineration methods, on the other hand, frequently lead to high expenditures for cleaning the exhaust gases of the pollutants and of the auxiliary fuel. The normally high incineration temperatures result in extremely high energy expenditures if the incineration warmth is not utilized.
All of the stated methods and apparatus in the art have certain disadvantages in their scope of application, service life, and the resulting total space-time yield, significantly reducing their cost-effectiveness.
Taking the shortcomings of other methods into consideration, it is the object of the present invention to provide a method and apparatus for carrying out the invention which circumvent the stated disadvantages, are more universally applicable, allow the resulting products to be easily removed from the system, and allow the mineralization of even difficult chemical substances, such as inert halogen-carbon compounds, halogenated hydrocarbons, or SF.sub.6.